This invention relates to a method of treating waste water containing a harmful ion such as a heavy metal ion or fluorine ion to remove the harmful ion by coagulating sedimentation by using an inorganic coagulant such as an iron salt or an aluminum salt.
For the treatment of waste water containing a harmful ion such as a heavy metal ion or fluorine ion, a conventional method is the coagulating sedimentation method using an inorganic coagulant. That is, under an appropriate pH condition a coagulant such as an iron salt, aluminum salt or magnesium salt is added to the waste water to form a hydroxide sediment in which the harmful ion is incorporated by adsorption, coprecipitation and/or reaction with the coagulant, and then the sediment is separated from the treated water. In the case of waste water containing fluorine, another conventional method is the addition of a calcium compound to precipitate fluorine as calcium fluoride. A problem common to these conventional methods is that the hydroxide sediment or calcium fluoride precipitate becomes sludge which is large in quantity and high in water content and which is harmful waste.
To decrease the quantity of sludge produced by the treatment of waste water, there are some proposals of reusing a portion of the sludge as coagulant for treating waste water For example, JP-A 60-241988 relates to the treatment of waste water containing fluorine with a calcium compound and an inorganic coagulant such as aluminum sulfate and proposes to reuse at least a portion (e.g., 20 to 50%) of the sludge as part of the coagulant. However, in this method the coagulating capability of the fresh coagulant is adversely affected by fluorine existing in the reused sludge. Therefore, the efficiency of the treatment is lowered, and it is difficult to greatly decrease the quantity of produced sludge.
JP-B 58-13230 proposes to treat waste water containing fluorine by the steps of first removing most of fluorine as calcium fluoride precipitate, then forming magnesium hydroxide precipitate which incorporates therein the remaining fluorine, separating the composite precipitate from treated water, dissolving the separated precipitate in acidic water and recycling the obtained solution. JP-A 1-107890, which relates to the treatment of waste water containing boron and fluorine by the addition of a calcium compound and an aluminum compound, proposes to reuse aluminum in the sludge by eluting aluminum from the sludge separated from treated water and recycling the aluminum eluate. By these methods it is possible to decrease the quantity of produced sludge to some extent, but it is difficult to greatly decrease sludge because it is difficult to dissolve most of magnesium or aluminum in the sludge. Besides, a large quantity of acid or base is consumed to dissolve magnesium or aluminum.